This invention relates to deposition of material from a gas phase onto a substrate surface and, more particularly, to generation of a source gas in proximity of the point of deposition. The deposition may be induced by a laser source of energy.
The deposition of materials from a gaseous phase onto a substrate surface is well known. Indeed, a common technique employs photolithographically produced masks for generating patterned deposits for use in fabricating integrated semiconductor devices. This technique requires multistep processing on a microscopic level. Typically, the technique provides a resolution which is limited by the wavelength of the exposure light, the resolution of the masks, the control and registration by which the masks are positioned during the process, and the subsequent etching and deposition techniques. Thus, the special resolution of the final product is limited by the ability to register and expose the masks, control the mask making process, and control the etching and deposition procedures employing the masks.
To overcome the problems associated with use of masks, methods of directed writing with lasers for deposition on substrates has been proposed. For example, Deutsch et al., U.S. Pat. No. 4,340,617 discloses a method for forming a thin film of material on the substrate surface by a photolytic decomposition of a gaseous compound of the material by laser source energy. The apparatus includes a gas deposition chamber with a jig for supporting the substrate together with a laser source of energy operating at a desired wavelength and optical system for focusing the energy toward the selected surface and at a position adjacent to the selected surface. A gaseous compound of the material to be deposited is introduced into the chamber, and the compound absorbs a portion of the incident laser energy at the selected wavelength which causess photo decomposition of the compound close to the surface, thereby releasing the material for a deposition upon the surface. Organo-metallic gases such as trimethylaluminimum and dimethylcadmium can be photolytically decomposed at wavelengths less than 260 nm. to form aluminum and cadmium deposits. Activation may be by a frequency doubled and continuous wave argon-ion laser operating at 514.5 nm. or, alternatively, a pulsed argon fluoride excimer laser at 193 nm. The photo deposition may occur primarily by photolysis of the van de Waals molecular films on the substrate surface (Brueck et al., 48 Physical Review Letters 1678 (1982)) or in the gaseous phase (Wood et al. 42 Applied Physics Letters 408 (1983)).
Others have employed laser source energy for pyrolytic decomposition of the gaseous compound by heating the substrate surface with the laser source energy. For example, Herman et al., Materials Research Society Symposium Proceedings Volume 17 (1983) briefly compare the ultimate limit on the rate of metal deposition by photolytic and pyrolytic processes and conclude that the low photolytic cross section for a compound such as trimethylaluminum indicates that the pyrolytic decomposition will result in deposition of aluminum metal at approximately three orders of magnitude faster than photolytic decomposition for laser intensities at acceptable levels. Herman also used dimethylcadmium, nickel tetracarbonyl and dimethyl zinc, running, approximately ten Torr of metal alkyl or carbonyl in combination with 700 Torr helium as a buffer.
Others have found need for laser source energy induced deposition or etching in order to fabricate integrated circuits totally by direct writing using a laser beam focused on the substrate surface. For example, Herman et al. Materials Research Society Conference (Boston, Nov. 15, 1983) use tungsten hexafluoride to deposit tungsten, nickel tetracarbonyl to deposit nickel, silane to deposit silicon, phosphine to dope polysilicon, hydrogen chloride and chlorine to etch silicon, and hydrogen fluoride and silicon to remove silicon dixoide, all induced by laser beam pyrolysis, to fabricate a MOS transistor by direct writing.
In all of these prior art uses of laser source energy for photolysis or pyrolysis to effect deposition of a material from a gaseous phase, the material is introduced into the deposition chamber already in compounded form (e.g. aluminum is pumped in as the gaseous trimethyl, and nickel may be introduced as liquid nickel carbonyl which has a partial pressure of several hundred Torr at room temperatures). Introducing source gases directly into the deposition chamber is greatly complicated by the fact that organo metallic compounds are generally highly toxic and sensitive to moisture and air. Also, the fact that only relatively stable compounds can be used limits the choice of materials that can be deposited. In addition, some desirable materials are not commercially available in suitable gaseous form of sufficient purity. In particular, the previously noted small cross section of trimethylaluminum deters use of photolytic deposition of aluminum. Also, the variety of materials that are needed during a single direct writing fabrication process may require many different compounds to be introduced and removed from the deposition chamber, and this presents different handling problems, especially with a volatile liquid such as nickel carbonyl. Thus, there is a need for a flexible and in situ generation of gaseous compounds of the materials to be deposited.